Viral diseases are responsible for both global pandemics and yearly seasonal epidemics, such as influenza. Outbreaks may be characterized by potentiated virulence and may occur suddenly, resulting in serious mortality. Importantly, viral diseases are not limited to humans. For example, influenza also affects livestock and birds, which may have significant impact on food supply in addition to increasing the risk of transmission to humans.
Influenza viruses contain viral RNA molecules enclosed within an envelope comprised of a matrix protein and a lipid bilayer. Embedded into the lipid membrane are glycoproteins known as HA (hemagglutinin) and NA (neuraminidase). HA is responsible for the binding of the virus to the host cell through sialic acid receptors, and NA acts to release virions from plasma membranes of the infected cells and allows the progeny virions to infect other cells spreading the infection. The HA and NA are also important in the immune response against the virus; antibodies against them may protect against infection. Neuraminidase is the target of many antivirals, such as oseltamivir, peramivir and zanamivir.
Within the interior of a virion, the genome of the influenza A virus has an RNA-dependent RNA polymerase, which is a heterotrimeric complex of three subunits (PA, PB1 and PB2). The RNA polymerase catalyzes viral RNA transcription and replication. Because transcription and replication of the virus depends on the activity of RNA polymerase, the enzyme is also a potential target for development of new anti-viral compounds, especially in light of the emergence of drug resistant viruses.